One problem in the above-described NMT-measuring is that the final measuring result is affected by a rather large number of factors, including a stimulation electrode junction, a path from dermal surface to nerve, a neuromuscular junction forming the actual target to be examined, the electric acitivity of a muscle, a path from muscle to dermal surface, the EMG-electrode junction. In particular, the electrode set-up and the fastening and state of electrodes may lead to errors in a measuring result, which are difficult to detect and/or whose effect on a measuring result is difficult to observe.
The electrical stimulation itself results in a certain additional block, a so-called stimulus artefact (or S.A.), which is the part of a stimulus not passing via the nerve-neuromuscular junction-muscle route but, instead, is electrically conducted directly from stimulus electrode junction to EMG-electrode junction. Since this direct electrical conductance proceeds faster than the passage of a stimulus through a nerve, the block caused by stimulus artefact appears in measuring results respectively prior to the actual muscular blcok caused by stimulation. Thus, in the prior art NMT-measuring methods, a stimulus artefact has been tried to be eliminated by using gate circuits in a manner that the block signal is mainly recorded only during the actual muscular block (see e.g. the article H.S. Lam, N.M. Cass, K.C. Ng, Electromyographic monitoring of neuromuscular block, Br. J. Anaesth. (1981), 53, 1351). In practice, however, a stimulus artefact and an actual muscular block may be partially overlapped, whereby such procedure is of no help but the measuring results will be distorted.